As known, an electret microphone consists of a charged and/or polarized polymer film, called an electret film, one surface of which is coated with a thin metal layer which constitutes one of the electrodes in the microphone. The electret film and the metallized electrode are biased having the metallized layer turned upwards along a rear plate which constitutes the second electrode. Between the electret film and the rear plate or the rear electrode a small air gap, called an air film, is formed and an electrostatic field between the two electrodes is created. This field is called a bias field. Upon acoustic influence by sound pressure in front of the microphone, the electret film vibrates and the height of the air gap is changed, whereby the electrostatic field is changed. Due to this variation of the electrostatic field, a varying voltage drop across the two electrodes arises. Thus, if an amplifier having a suitable impedance match is connected between the two electrodes a voltage change depending on the variable sound pressure on the electret film can be obtained.
Since the voltage variation across the two electrodes is weak, it is, as above mentioned, necessary to connect an amplifier with a suitable impedance match to the electrodes. The problem is then to limit the number of parts in the microphone and at the same time to have a good connection between electret and amplifier.
It is previously known for this purpose to build in the electret microphone an integrated circuit containing a preamplifier and to form the same into a single capsule as shown, for example, in the Bell System Technical Journal, September 1979, No. 7, page 1557. Thereby a compact unit is obtained.